feat: Support Terraform remote state when generating GCP resources

[adlersantos]

Description

Based on #33. We need to optionally support Terraform remote state configuration via a backend.tf file that's auto-generated. It contains the following:

terraform {
  backend "gcs" {
    bucket = "REMOTE_STATE_BUCKET"
    prefix = "REMOTE_STATE_PREFIX"
  }
}

This allows users to store a file on GCS representing a record of the GCP resources that Terraform keeps track of for their pipelines.

It's now possible to do this via the --tf-state-bucket and --tf-state-prefix arguments for scripts/generate_terraform.py.

Because backend states differ per user per environment, we only create the backend.tf file in the env folder specified, e.g. .dev/datasets/ml_datasets/_terraform/backend.tf. That is, we don't check in Terraform remote state files in the repo to keep it optional for everyone.

Checklist

• Tests pass.
• Linters pass.
• Please merge this PR for me once it is approved.
• If this PR adds/edits/deletes a feature, I have updated the README accordingly.